Technical Field
The present invention relates to a lubricating oil composition and a transmission fluid for vehicles which uses the lubricating oil composition.
Background Art
Lubricating oils have been inherently used for the purpose of reducing friction at sliding portions by forming an oil film on the sliding portions. Therefore, in order to form a strong oil film, it is advantageous that the lubricating oils have a high viscosity. On the other hand, as the viscosity of the lubricating oils becomes higher, an amount of power required upon stirring the lubricating oils or supplying the oils to lubricating parts is increased to thereby cause a large energy loss in a power engine and deterioration in fuel consumption. In consequence, in recent years, reduction in viscosity of the lubricating oils has proceeded to reduce a power loss and improve saving of energy and reduction in fuel consumption.
However, if the viscosity of the lubricating oils is excessively reduced, it becomes difficult to form an oil film on sliding portions when exposed to a high temperature. As a result, there tend to occur increase in friction at the sliding portions and abnormal abrasion thereof.
For this reason, in order to achieve both oil film formation under high temperature conditions and reduction of an energy loss in a normal temperature range or lower, it is considered effective to reduce a viscosity of the lubricating oils in the normal temperature range or lower while maintaining a high viscosity thereof under high temperature conditions. This means that the change in viscosity of the lubricating oils depending upon a temperature change is extremely reduced, i.e., there is a large demand for lubricating oils having an extremely high viscosity index (high VI).
To solve the above problem, there have been used lubricating oils that are improved by compounding an additive therein. For example, PTL1 to PTL3 have proposed lubricating oil compositions including a viscosity index improver as the additive.